Magnetic resonance imaging (MRI) is a medical imaging modality that can create pictures of the inside of a human body without using x-rays or other ionizing radiation. MRI uses a powerful magnet to create a strong, uniform, static magnetic field (i.e., the “main magnetic field”). When a human body, or part of a human body, is placed in the main magnetic field, the nuclear spins that are associated with the hydrogen nuclei in tissue water become polarized. This means that the magnetic moments that are associated with these spins become preferentially aligned along the direction of the main magnetic field, resulting in a small net tissue magnetization along that axis (the “z axis,” by convention). An MRI system also comprises components called gradient coils that produce smaller amplitude, spatially varying magnetic fields when a current is applied to them. Typically, gradient coils are designed to produce a magnetic field component that is aligned along the z axis, and that varies linearly in amplitude with position along one of the x, y or z axes. The effect of a gradient coil is to create a small ramp on the magnetic field strength, and concomitantly on the resonant frequency of the nuclear spins, along a single axis. Three gradient coils with orthogonal axes are used to “spatially encode” the MR signal by creating a signature resonance frequency at each location in the body. Radio frequency (RF) coils are used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei. The RF coils are used to add energy to the nuclear spin system in a controlled fashion. As the nuclear spins then relax back to their rest energy state, they give up energy in the form of an RF signal. This signal is detected by the MRI system and is transformed into an image using a computer and known reconstruction algorithms.
As mentioned, radio frequency (RF) coils are used in an MRI system to transmit RF excitation signals and to receive MR signals emitted by an imaging subject. Various types of RF coils may be used in an MRI system such as a whole-body RF coil and RF surface (or local) coils. Two common RF coil configurations are the birdcage coil and the transverse electromagnetic (TEM) coil.
During an MRI scan, acoustic noise and vibration can be generated in the patient bore. The acoustic noise and vibration can be uncomfortable and potentially harmful to both the patient and scanner operator. There are several sources of acoustic noise in an MRI system including, for example, the gradient coils and the RF coils. The acoustic noise generated by the RF coil is typically caused by eddy currents induced in the RF coil conductors by operation of the gradient coils. In particular, current pulses are applied (e.g., as part of a pulse sequence) to the gradient coils to generate time-varying magnetic fields. These time-varying magnetic fields can induce eddy currents in an RF coil that cause motion or vibration of the RF coil and result in acoustic noise. In addition, the eddy currents induced in the RF coils can produce heating. The heat produced by the RF coils can cause an increase in the temperature of the patient bore which can affect patient comfort and the efficiency of the MRI system.
It would be desirable to provide an RF coil configured to reduce or eliminate heating, vibration and acoustic noise generated by the RF coil. It would also be desirable to provide an RF coil configured to produce a uniform excitation (B1) field.